The present invention relates to electrophotographic imaging processes involving development and fixation of toner images wherein the resinous component of the electrostatographic toner comprises a block or graft copolymer having crystalline and amorphous segments.
The formation and developement of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This toner image may then be transferred to a copy substrate such as paper. The transferred image may subsequently be permanently affixed to the copy substrate such as by fusion with heat. Instead of forming the latent image by uniformly charging the photoconductive layer and then exposiing the layer to a light and shadow image, one may form the latent image by directly charging an insulating layer, which can be either photoconductive or non-photoconductive, in image configuration. The powder may be fixed directly to the insulating layer if desired.
One of the important applications of electrostatography comprises its use in automatic copying machines for general office use wherein an electrostatic latent image is developed using a developer composition comprising a carrier mixed with fine particles of resinous toner, and the thus formed powder image is transferred to a copy substrate and then fixed thereon. Considerable effort has been expended to provide suitable developers and associated fixing techniques for modern high speed copying machines. The toner material used must have suitable electrostatic properties to permit attraction by the carrier and then selective attraction by the latent images. It must further be physically strong to permit constant recycling in a bouncing type of movement. The toner must further be resistant to blocking or aggregating at ordinary operating temperatures, but yet be capable of being readily fixed to the copy sheet.
Fixing techniques employing heat, pressure, solvents and various combinations thereof have been devised; however, each of these systems is subject to severe practical limitations which inhere in the systems themselves and also the toner compositions heretofore available. Whatever method of fixing is used, speed, effectiveness, and simplicity in operation are the principal, desirable characteristics to be obtained. The most commonly employed fixing techniques employ the use of heat alone or heat in combination with pressure. The toner materials employed must melt or block sufficiently above the ordinary operating temperatures of the machines involved to assure convenient storage and handling. However, the materials must also melt at a practically low temperature to avoid excessively-high energy consumption and possible heat damage to the copy substrate or delicate machine parts.
It has long been recognized that one of the fastest and most positive methods of applying heat for fusing the powder image to paper is to bring the powder image into direct contact with a hot surface, such as a heated flat plate or roller. However, it was found that as a powder image is tackified by contact heating, part of the image carried by the copy sheet would stick to the hot surface so that as the next copy sheet was contacted with the hot surface, the tackified image partially removed from the first sheet would partially transfer to the next sheet and, at the same time, part of the tackified image from said next sheet would adhere to the hot surface. This phenomenon is commonly referred to in the printing art as "offset". For a given system, this upper temperature limit is referred to as the "hot offset temperature". Thus, contact heat fusers are inherently limited to the use of temperatures and toners which do not cause hot offset of the toner material.
Various types of polymeric materials have been proposed in the prior art for use as the resinous component in electrostatographic toners. U.S. Pat. No. RE 25,136 teaches toner material based on polystyrene or copolymers of styrene with monomers such as alkyl methacrylates. British Pat. No. 1,179,095 teaches toner material based on a combination of two polymeric materials, one of which polymeric materials has a glass transition temperature of greater than 20.degree.C, and the other a glass transition temperature of at least 5.degree.C lower. These polymeric materials may be combined by physical admixture, or by forming block or graft copolymers. Other patents of interest in the electrostatographic toner area include U.S. Pat. Nos. 2,788,288; 3,078,342; 3,391,082; 3,502,582; 3,510,338; 3,609,082 and 3,647,696. As indicated above, these other toner materials based on amorphous polymers have not proven entirely satisfactory when used with contact heat fusers.
While further advances in the art of fixing, including the use of offset reducing roller surfaces, have provided more suitable means to fuse toner images through the use of heat and pressure with decreased offset, these devices are still restricted to operate within close temperature tolerances due to the narrow fusing latitudes obtainable with toner materials heretofore available. Exemplary of these contact fusing devices are those disclosed in U.S. Pat. Nos. 3,256,002, 3,268,351, 3,291,466, 3,437,032, 3,498,596, and 3,539,161.